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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

184

CHROME STEELS

Mfs

the titration. The volume of thiosulphate used corresponds with 0-09806
gram of potassium dichromate, and hence with 0-03466 gram of chromium.

(a) DETERMINATION OF THE CHROMIUM IN ABSENCE OF VANADIUM
AND MOLYBDENUM. 2 grams of the sample, in a small porcelain dish
covered with a clock-glass, are dissolved in nitric acid of D = 1-18 and the
solution evaporated to dryness and the residue ignited to decompose the
nitrates. When cold, the oxides are transferred quantitatively with a
platinum spatula to an agate mortar in which they are mixed with sodium
peroxide, the dish being repeatedly cleaned in the dry with small quantities
of the peroxide (of which not more than 10-15 grams should be used alto-
gether). The last traces of oxides adherent to the dish are removed by
heating with a little dilute sodium hydroxide solution, the alkaline solution
together with the water used to wash out the dish being transferred to a
beaker.

The peroxide mixture is heated gently to fusion in a covered nickel or
porcelain crucible, the heating being then continued for about 15 minutes
at dull red heat. The cold crucible is treated in a covered beaker with
hot water in which the mass rapidly dissolves, the alkaline liquid used for
the final washing of the dish being then added, the crucible withdrawn
and washed and the beaker kept for 1-2 hours on a steam-bath. If the
solution appears greenish, owing to the presence of manganates, the latter
are reduced by addition of a little sodium peroxide. The liquid is then
made up to about 200 c.c., allowed to cool and filtered by decantation, the
filtrate being collected in a 500 c.c. flask and the residue washed repeatedly
with dilute sodium carbonate solution.1 When the washing is completed,
the filtrate is boiled to decompose any traces of sodium peroxide, still present
and, after cooling, made up to volume.

To 100 c.c. of this solution (0-4 gram of the sample), acidified with hydro-
chloric acid, are added 10 c.c. of 10% potassium iodide solution, the liquid
being then diluted to 200-300 c.c. and the free iodine titrated with thio-
sulphate as before.

(6) DETERMINATION OF THE CHROMIUM IN PRESENCE OF VANADIUM
AND MOLYBDENUM. When the oxidation of the chromium by fusion with
sodium peroxide (see a, above) is complete, the aqueous extract—which
may contain alkaline vanadates and molybdates in addition to chromate—is
neutralised exactly with nitric acid (towards methyl orange), heated to
boiling and treated with a slight excess of 10% mercurous nitrate solution.
Under these conditions the chromium, vanadium, molybdenum, tungsten
and phosphorus are precipitated.

The liquid is heated to boiling and, when the precipitate has settled,
mercurous nitrate solution is added to ascertain if the precipitation is com-
plete. In case the solution has become acid, it is neutralised with a few
drops of ammonia and filtered, the precipitate being washed first with
water and then with very dilute mercurous nitrate solution. The filter
and the whole of the precipitate, in an open platinum crucible, are then

1 If traces of chromium still remain in the residue, the latter is fused with sodium
and potassium carbonates, the resulting product extracted and the new solution added
to that of the first treatment.etween zinc sulphide and iodine takes place thus :   Zn S + 2! =